Gas electrodes in which a gas is contacted with an electrode in the presence of an electrolyte solution for electrolysis are well known. In their typical uses, gas electrodes function in systems capable of generating electricity (such as fuel cells and the like) or for electrolysis purposes in which the electrode performs as a depolarized electrode (as in chlor-alkali and the like). Gas electrode installations implement electrochemical reactions involving the interaction with, and between, three reactants: (1) a gas; (2) a liquid (usually aqueous) electrolyte; and (3) electrons, all of which require simultaneous contact in order to accomplish a desired reaction.
Depolarized electrodes generally have two oppositely disposed vertical faces. One of the faces is adapted to contact a liquid electrolyte, while the other face is adapted to contact a gas. During operation of the cell, the liquid permeates into the interstices of the electrode from one side, while the gas permeates into the interstices of the electrode from the other side. Inside the electrode, there is formed a three phase contact between the liquid electrolyte, the gas, and the solid electrode body. Electrical energy is transferred into the electrode and electrolytic reactions occur. Such electrical reactions could be electrolysis reactions to produce products such a chlorine, hydrogen chloride, or caustic. Optionally, such reactions could be conducted to produce electrical energy (as in a fuel cell or a battery) rather than consume electrical energy.
Oxygen depolarized electrodes are of special interest in commercial, large scale chlor-alkali operations and analogous electrolyzers of other alkali metal or acid halides. In the electrolysis of common salt brine, for example, the reaction at the depolarized cathodic oxygen electrode in the alkaline media of the catholyte is: EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- with E.degree.=+0.401 volts
In comparison, the cathode reaction in a traditionally conventional chlor-alkali cell is: EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- with E.degree.=-0.828 volts
Thus, the use of an oxygen depolarized cathode for chlor-alkali electrolytic cells brings about a theoretically achievable electrical potential requirement saving of 1.229 volts. This, for practical purposes, translates to the possibility of very substantial reduction in and economizing of power costs, when compared with the non-depolarized chlor-alkali cells.
Depolarized electrodes can also function as depolarized anodes. Depolarized anodes have a hydrogen-containing gas contacting one side of the anode and an electrolyte contacting another side. In the case of chlor-alkali cells, hydrogen gas may be contacted with one side of the anode and a sodium chloride brine solution contacted with another side of the anode to produce hydrogen chloride.
Although such electrodes are theoretically very useful, there are certain considerable difficulties involved in their use in large-scale commercial manufacturing purposes (as in the chlor-alkali trade). Three major problems include: the leaking of gas or electrolyte across the gas-permeable electrode; the difficulty of distributing the electrical current relatively evenly to all the active surface of the electrode; and the difficulty of supporting such electrodes to prevent their rupture.
A significant and perplexing problem is the frequent occurrence of the passage of reactant gas through the electrode and into the electrolyte in tall cell assemblies. In many commercial installations, the electrolyte is often contained in contact with the electrode in considerable depth (frequently as deep as 4-6 feet, or deeper). With a liquid hydraulic pressure of such magnitude, the catholyte exerts a substantial hydraulic pressure on the face of the electrode. If the gas pressure is reduced to avoid bubbling in the upper portions of the electrode, the increasingly pressurized liquid towards the lower electrode portions overcomes the restraint of the applied gas and leaks into the gas chamber. This often tends to render inoperable, or at least considerably diminish, the effectiveness and productive capacity of the cell. Leaking of gas or electrolyte through the electrode is, therefore, extremely undesirable. Not only does it tend to materially interfere with and diminish overall reaction efficacy, it occasions, among other things, escape of reaction gas which is either lost or, if collected, must be handled through recovery and reprocessing units for subsequent reuse. In any event, leaking can increase the cost of the operation. The present invention provides an article which minimizes the leakage of gas and electrolyte through depolarized electrodes.
A second problem encountered with the use of large-scale depolarized electrodes involves the difficulty in obtaining uniform distribution of current to the entire active surface of the electrode without blocking substantial portions of the electrode to the gas. Uniform distribution of current to the electrode requires a large number of electrical connections which, when attached to the electrode, tend to prevent gas from gaining access to the electrode. The present invention provides an article which provides means to assure a somewhat uniform distribution of electrical current to the surface of depolarized electrodes without blocking substantial amounts of gas from entering the electrode, would be highly desirable.
A third, and perhaps even more significant, problem in using large scale depolarized electrodes involves the difficulty in supporting the depolarized electrodes against large liquid hydraulic pressures while not blocking gas from the surface of the electrode. The large liquid hydraulic pressures encountered in tall electrodes frequently require extra support in the lower portions of the vertically disposed electrode to minimize the likelihood of the electrode rupturing. The present invention provides an article which provides the necessary support for tall depolarized electrodes without substantially interfering with the delivery of gas to the surface of the electrode.